List of artists that have never lost quality throughout entire career

I agree with @schubert , Bach was an one of a kind genius. His compositions are complex and beautiful. Anybody who plays the piano would tell you the same. 

Of course a rock.pop, country "artist" requires little to no talent or musical knowledge .

Eddie Van Halen -- heavy metal, hard rock artist classically trained on acoustic guitar

Steve Vai -- jazz, jazz-rock, electronic rock multi-genre artist was classically trained on acoustic guitar.

Andy Summers -- jazz, rock, wave guitar artist former Police guitarist was classically trained by one of the greatest classical guitar maestro Emanuel Barrueco

Jan Akkerman -- classically trained on violin performs as lead guitar of jazz-rock band Focus

Jack Bruce -- classically trained on violin and performed as his most know act at Cream -- blues-rock power trio

Joey DiFrancesco -- jazz organist, jazz-rock -- self-trained since the age of 3 and performing since age of barely 7 at the stage (is there any talent?)

Irmin Schmidt -- electronic rock, kraut rock keyboardist and pianist -- alumni of Stockhousen Conservatory as pianist and orchestral conductor. Stockhousen Conservatory is school of innovative music and Jazz in Germany, but it also provides classical training. One of the most unusual schools of music in the world, but bare in mind that classical music rootes mainly from Germany and Austria and I believe still continue to add on...

There are many and many more rock or other ganre artists that literally infused J.S.Bach in their tunes and improvisations -- YES indeed he's one of the greatest, because his music still influences the most current tunes you hear -- just need to realize and just need to optimize judgments towards definitions of talent and musical knowledge. 

Country artists vary between family musical training and various schools of art. I may not find myself enjoying most of the country music, but I do find myself respectful to country artists skill to play instrument. When I hear Ricky Scaggs reefing acoustic guitar, I want to make it lowder, but when I hear his typical country style voice, I may want to shut it all the way down:-)

Classical music even nowdays still evolves, but not finding large enough listener due to commercial media. There are still classical composers of today finding their place mostly in the movie soundtracks. One of my favorite is Maurice Jarre.

Just saw this thread. In regards to new wave music only. I really like the old Depeche Mode but after Alan Wilder left they frankly have been making just childish crap.  DM's music has loss it's soul and the lyrics are simply childish.  With that said, the newly released 101 album on vinyl is fantastic. Some of the bands I think that have tested time are Radiohead, Blur, Tears for fears, U2, Johnny Marr and Echo and Bunnymen.  My taste in music is wide and vast.  Been listening to a lot jazz lately. Wish Coltrane and Monk were still alive. 

Drag, a poor performance of Back has nothing to do with Bach himself.
Of course a rock.pop, country "artist" requires little to no talent or musical knowledge .

I’ve heard all the Brandenburg's played at least a hundred times live, never heard a bad performance yet . Neither has anyone else who goes to the best orchestras.

No one has ever said Bach was the greatest organist, all comments made in his life-time did said he was a master .
The most agreed upon  opinion among serious lovers of Classical Music worldwide is that he is the greatest composer who ever lived or ever will live .

Tom Petty than is fraction of Willie Nelson who’s career is way over 40 years.

i've never been a massive tom petty fan, though he's had a ton of great singles, but i gotta say that (unlike say, dylan, or the stones) his output has been impressively consistent throughout a 40-year career + he still does it live

Haven't you ever heard the Brandenburg played badly? Sounding like a demented circus calliope on methedrine?

It's a good piece, but it can easily be butchered.

With new technology today and new training programs available for musicians, I can say that there are plenty of organists that could possibly outplay J.S.Bach even playing his own pieces. Since we can't hear J.S.Bach recorded, we can't really judge...
Remember, history is never precise

I can , J.S. Bach .

I can't think of a single artist, in any genre, that hasn't put out at least one dog. In the long term, anyway.

A good friend of mine, now deceased, used to say:

"Every Beatles album has its "Octopus's Garden".

Elvis Costello has never disappointed.

Radiohead

Wilco

Tom Waits

Grateful Dead (OK, debatable, but I'm a die hard!)

Oh yes, thank you, definitely Billie Joe Shaver, an under appreciated singer, writer AND performer.  Awesome in concert.

Billy Joe Shaver

Just saw Emo Phillips on a Showtime thing…I had no idea he was still around, and hilarious.

Another vote for Depeche Mode, and I would add Duran Duran.

Yes, but solo he has a few clunkers. Their are many great artists and bands but few that little or no clunkers.

Pete Townshend has been doing it for 50 years now. Both with The Who and solo. 

Steely Dan, Frank Zappa, Bob Dylan, Oscar Peterson, Dizzy Gillespie, Ry Cooder, Dire Straits, Mark Knopfler, Willie Nelson, Beatles, Miles Davis, Led Zeppelin, Talking Heads, Frank Sanatra, Joni Mitchell, K.D. Lang, CCR, There are many more but this just came off top of my head.

Some really big names, some my favorites I've left off because the have a few so, so or not so good albums.

I can't say I've really followed an artists whole career except for the Grateful Dead.  But, here are a few that as of now have not let me down. 
Zappa
Neko Case
Dan Hicks
Brand X
Gentle Giant

I almost forgot Jimi, start to finish. Of any artist I can think of his was a star I can not help to feel saddened by the what if's

+1  Leonard cohen

david byrne

Robert Plant, in all forms!

Agree about Springsteen. Also my personal favorites: Paul Simon and, especially, Leonard Cohen. Cohen's last concerts were masterful and kept getting better.

DBTOM:

George was an older more couth version of Rodney Dangerfield.

He mostly talked and joked, but there was a little music there if you hung around long enough.

I loved watching George Gobel when I was a kid, so somewhat jokingly mention him in these forums whenever I feel the levity of such is appropriate (think he may even get a kick out of the posts).


DeKay

..Louis Armstrong as mentioned briefly.

Several great classical musicians

 e.g. Pablo Casals

Peter Gabriel!

In Bruce Springstin I saw and heard everything: arrangements, good release management, sound, mastering, good live concert management, but, unfortunately, no music! It seems always to me that music isn't Bruce's cup of tea at all. Song "Born In The USA" heard on car radio triggers my wrist to change station few seconds after it's heard.

Excellent call on Van Morrison.

I’d include Andy Summers (especially, post-Police) and Neil Finn/Crowded House.

Quality = Van Morrison

+1 for Depeche Mode!  

Bruce Springsteen is an artist whoose work has matured and grown.  lyrically  he's become more complex as well.   I heard some of his recent shows where's he's performed Born in the USA and The River.   I not only saw this music performed in the era of its release, but have recently heard recordings of these old shows. 
His band's  new performance of this old material show an incredible musical advancement over the old shows.   The arrangements are similar but are so much more masterful. 

@dekay 

Haven't thought of George Gobel in decades but when you mentioned him, this quote came to mind:

"Did you ever get the feeling that the world is a tuxedo and you're a pair of brown shoes?"

Yes. I have. 

George Gobel.

All others pale in comparison.

DeKay

Quality = Captain Beefhart

I have the same feeling for Steve Howe.
He's probably got arthritis affecting his wrists and he can't play clean anymore as he used to play
He doesn't trip, but has trouble keeping sound clean against frets. It happens with aging musicians. Andres Segovia closer to the end of his career also sounded bad.